Subquery and JOIN2

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Revision as of 09:42, 30 July 2012 by Connor (Talk | contribs)

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Converting aggregate subqueries into Join functions.

schema:scott
DROP TABLE orders
 CREATE TABLE orders(
  employee VARCHAR(40)),
  whn DATE(YYYY,MM,DD),
  totalitems INTEGER));
INSERT INTO orders VALUES ('Jim',(2006,10,10), 5);
INSERT INTO orders VALUES ('Jim',(2006,10,11), 3);
INSERT INTO orders VALUES ('Jim',(2006,10,12), 1);
INSERT INTO orders VALUES ('Brian',(2006,10,10), 7);
SELECT customer, whn, totalitems
FROM orders 01
WHERE 01.whn = (
 SELECT MAX(whn)
 FROM orders 02
 WHERE 01.customer = 02.customer)

To make this more efficient a HAVING clause can be used with a self join.

SELECT 01.customer, 01.whn, 01.totalitems
FROM orders a JOIN orders b ON (a.customer = b.customer)
GROUP BY a.customer, a.whn, a.totalitems
HAVING a.whn = MAX(b.whn)
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